Maleimide terminated rubber and curable composition produced by using the maleimide terminated rubber

ABSTRACT

An object of the present invention is to provide a curable composition which exhibits excellent toughness after the curing and excellent long term stability, as well as a rubber used in such curable composition. Such object is realized by a maleimide terminated rubber having maleimide structure on both ends of the molecule obtainable by reacting a rubber having amino group and/or imino group on both ends of the molecule and a bismaleimide compound, as well as a curable composition comprising such maleimide terminated rubber and a resin and/or a rubber other than such maleimide terminated rubber.

TECHNICAL FIELD

This invention relates to a curable composition which exhibits excellentlong-term stability as well as toughness after curing, and a rubber usedin such curable composition.

BACKGROUND ART

A liquid rubber having amine structure on both ends of the molecule(hereinafter referred to as an “amine terminated rubber”) has been knownin the art (see, for example, B.F. Goodrich Chemical Company, TechnicalData and L. C. Chan, J. K. Gillham, J. Kinloch, Polymer, 24, 1341,1355(1983)). These amine terminated rubber has a rubber backbone and areactive amine structure on both ends of the molecule, and therefore,these rubbers have been used for the purpose of imparting toughness (theproperty which enables bending without fracture) to a thermosettingresin. For example, epoxy resin is used for various applications owingto its excellent dynamic properties (for example, modulus) and heatresistance. However, epoxy resin is insufficient in the toughnessrequired in some particular applications, and the amine terminatedrubber is used in such applications.

However, when the amine terminated rubber that had been known in the artwas used in combination with a thermosetting resin such as epoxy resin,the resulting composition exhibited insufficient long term stabilitysince the amine terminated rubber had low molecular weight, and theamine terminated rubber was highly reactive with a group such as epoxygroup.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a curable compositionwhich exhibits excellent toughness after the curing as well as excellentlong term stability, and another object of the present invention is toprovide a rubber used in such curable composition.

A further object of the present invention is to provide a compositemolded article produced by using the curable composition as describedabove.

The inventors of the present invention have made an intensive study onthe amine terminated rubber and the curable composition using the amineterminated rubber, and found that, when a bismaleimide compound isreacted with the amine structure of the amine terminated rubber forconversion of both ends into bismaleimide structure, toughness of thecurable composition can be improved simultaneously with the long termstability. The inventors also found that, when the curable compositioncontains epoxy resin, improvements in strength and other dynamicproperties as well as heat resistance are realized. The presentinvention has been completed on the bases of such findings.

The present invention provides (1) to (7) as described below.

(1) A maleimide terminated rubber having maleimide structure on bothends of the molecule obtainable by reacting a rubber having amino groupand/or imino group on both ends of the molecule and a bismaleimidecompound.

(2) A curable composition comprising the maleimide terminated rubber ofthe above (1) and a resin and/or a rubber other than the maleimideterminated rubber.

(3) The curable composition according to the above (2) furthercomprising a curing agent having a functional group capable of reactingwith the maleimide structure.

(4) The curable composition according to the above (3) wherein thefunctional group is at least one member selected from a group consistingof amino group, imino group, thiol group, and diene structure.

(5) The curable composition according to the above (3) wherein the resinis epoxy resin, and the functional group is at least one member selectedfrom a group consisting of amino group, imino group, and thiol group.

(6) A composite molded article wherein the curable composition of anyone of the above (1) to (5) is used.

(7) A prepreg wherein the curable composition of the above (5) is used.

The maleimide terminated rubber of the present invention has a rubberbackbone, a relatively high molecular weight, and maleimide structure onboth ends of the molecule which has relatively low reactivity at ambienttemperature. As a consequence, the curable composition of the presentinvention produced by using the maleimide terminated rubber of thepresent invention exhibits excellent toughness after the curing as wellas excellent long term stability, and such composition is highly adaptedfor use in applications such as composite molded articles. Among suchcurable composition of the present invention, those including epoxyresin are highly adapted for use in applications such as prepreg sincethe composition exhibits superior dynamic properties and heat resistancecompared to the compositions free from the maleimide terminated rubberof the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the present invention is described in detail. First, maleimideterminated rubber of the present invention is described.

The maleimide terminated rubber of the present invention is a rubberhaving maleimide structure on both ends of the molecule obtainable byreacting a rubber having amino group and/or imino group on both ends ofthe molecule with a bismaleimide compound.

The rubber having the amino group and/or imino group on both ends of themolecule used in the present invention is not particularly limited forits backbone. Exemplary rubbers include those having the amino groupand/or imino group introduced into both ends of the molecule having theskeleton of a conventional known diene rubber or its hydrogenationproduct (for example, natural rubber, epoxidated natural rubber,isoprene rubber, styrene-butadiene rubber, hydrogenatedstyrene-butadiene rubber, butadiene rubber (high cis butadiene rubber orlow cis butadiene rubber), acrylonitrile-butadiene rubber (NBR), orhydrogenated acrylonitrile-butadiene rubber), olefin rubber (forexample, ethylene-propylene rubber, ethylene-propylene-diene rubber,maleic modified ethylene-propylene rubber, butyl rubber, copolymer ofisobutylene or an aromatic vinyl or diene monomer, acrylic rubber, orionomer), halogen-containing rubber (for example, brominated butylrubber, chlorinated butyl rubber, brominated isobutylene-paramethylstyrene copolymer, chloroprene rubber, hydrine rubber, chlorosulfonatedpolyethylene, chlorinated polyethylenes, or maleic modified chlorinatedpolyethylene), silicone rubber (for example, methylvinyl siliconerubber, or methylphenylvinyl silicone rubber), sulfur-containing rubber(for example, polysulfide rubber), fluorocarbon rubber (for example,vinylidene fluoride rubber, fluorine-containing vinyl ether rubber, orfluorine-containing phosphazene rubber), urethane rubber, liquidpolyisoprene, liquid polybutadiene, liquid 1,2-polybutadiene, liquidstyrene-butadiene rubber, liquid polychloroprene, liquid siliconerubber, liquid fluorocarbon rubber, thermoplastic elastomer (forexample, styrene-butadiene-styrene block copolymer,styrene-isoprene-styrene block copolymer, styrene-ethylenebutylene-styrene block copolymer, or other styrene elastomers, olefinelastomer, ester elastomer, urethane elastomer, polyamide elastomer,polyvinyl chloride elastomer), or thermosetting elastomer (for example,urethane elastomer or silicone elastomer).

Among these, the preferred are the rubber wherein amino group and/orimino group is introduced on both ends of the NBR molecule. Exemplarysuch rubbers include commercially available rubbers such as HYCAR ATpolymer ATBN1300×16 manufactured by Ube Industries, Ltd. and ATBN1300×45manufactured by Ube Industries, Ltd. which is represented by thefollowing formula (1):

The bismaleimide compound used in the present invention is notparticularly limited, and conventional known bismaleimide compound maybe used. The preferred is the bismaleimide compound represented by thefollowing formula (2):

In the formula, R¹ to R⁴ independently represent a group selected from agroup consisting of —H, —CH₃, —C₂H₅, —C₃H₇, —F, —Cl, —Br, and —I; Xrepresents an optionally substituted divalent acyclic aliphatichydrocarbon group containing 1 to 24 carbon atoms; an optionallysubstituted cyclic aliphatic hydrocarbon group containing 5 to 18 carbonatoms; an optionally substituted divalent aromatic hydrocarbon groupcontaining 6 to 18 carbon atoms; or a group containing at least onemember selected from a group consisting of SO₂, O, N, and S in the abovementioned divalent group.

Exemplary X include simple bond and a group selected from the followingformulae wherein p and q are independently an integer of at least 1.

Among these, the preferred are a group selected from the followingformulae.

Exemplary bismaleimide compounds represented by the above formula (2)include 1,2-bismaleimide ethane, 1,6-bismaleimide hexane,N,N′-1,2-phenylenedimaleimide, N,N′-1,3-phenylenedimaleimide,N,N′-1,4-phenylenedimaleimide, N,N′-1,4-phenylene-2-methyldimaleimide,N,N′-(1,1′-biphenyl-4,4′-diyl)bismaleimide,N,N′-(3,3′-dimethyl-1,1′-biphenyl-4,4′-diyl)bismaleimide,4,4′-diphenylmethane bismaleimide,N,N′-(methylenebis(2-chloro-4,1-phenylene))bismaleimide,bis(3-ethyl-5-methyl-4-maleimide phenyl)methane, 2,2-bis(4-(4-maleimidephenoxy)phenyl)propane, N,N′- (sulfonylbis(1,3-phenylene)) dimaleimide,and N,N′-(4,4′-trimethyleneglycol dibenzoate)bismaleimide.

The bismaleimide compound used may also be a maleimide-modified polymercompound (for example, resin and rubber).

Among such compounds, the preferred are 1,6-bismaleimide hexane,1,2-bismaleimide ethane, N,N′-1,3-phenylenedimaleimide,4,4′-diphenylmethane bismaleimide, bis(3-ethyl-5-methyl-4-maleimidephenyl)methane, and 2,2-bis(4-(4-maleimide phenoxy)phenyl)propane inview of the cost.

The maleimide terminated rubber of the present invention may be obtainedby reacting at least one rubber having amino group and/or imino group onboth ends of the molecule and at least one bismaleimide compound so thatthe resulting compound has maleimide structure on both ends of themolecule.

In order to allow the reaction to proceed such that the resultingcompound has maleimide structure on both ends of the molecule, therubber and the bismaleimide compound are reacted such that the maleimidestructure of the bismaleimide compound and the amino group and/or iminogroup on both ends of the rubber molecule are at the ratio in equivalentof preferably 1.05 to 2, and more preferably 1.05 to 1.5.

More specifically, the maleimide terminated rubber of the presentinvention may be obtained by mixing the rubber having amino group and/orimino group on both ends of the molecule and the bismaleimide compoundas described above in the ratio as described above preferably in asolvent, and stirring the mixture at a temperature in the range of roomtemperature to 70° C. for 10 to 60 minutes. Typical solvents includemethyl ethyl ketone (MEK), acetone, and N,N-dimethylformamide.

This reaction is schematically represented by the following formula:(n+1)E-A-E+(n+2)J-G-J→J-G-J

E-A-E-J-G-J

_(n)E-A-E-J-G-J

In the above formula, A represents backbone of the rubber, E representsamino group, imino group, or a moiety derived from such groups, andE-A-E represents the rubber having the amino group and/or the iminogroup on both ends of the molecule. G represents the moiety of thebismaleimide compound other than the maleimide structure, J representsthe maleimide structure of the bismaleimide compound or a moiety derivedfrom the maleimide structure, and J-G-J represents the bismaleimidecompound. In each formula, the plurality of A, E, G, and J mayindependently represent two or more different moieties. n represents aninteger of 0 or more.

The maleimide terminated rubber of the present invention may comprisetwo or more types of molecules wherein n is different, or two or moretypes of molecules wherein at least one of A, E, G, and J is different.

The maleimide terminated rubber of the present invention obtainable bythe procedure as described above has maleimide structure on both ends ofthe molecule. The maleimide structure is represented by the followingformula (3).

In the formula, R⁵ and R⁶ are independently a group selected from agroup consisting of —H, —CH₃, —C₂H₅, —C₃H₇, —F, —Cl, —Br, and —I.

Among these, the combination of R⁵ and R⁶ is preferably —H and —H, or —Hand —CH₃.

The maleimide terminated rubber of the present invention is notparticularly limited for its molecular weight. However, the preferableweight average molecular weight is, for example, in the range of 1,000to 100,000.

The maleimide terminated rubber of the present invention may also be amixture of two or more rubbers each having different backbone, maleimidestructure, or molecular weight.

As described above, the maleimide terminated rubber of the presentinvention has a rubber backbone and maleimide structures on both ends ofthe molecule. Accordingly, when the maleimide terminated rubber of thepresent invention is used in the curable composition of the presentinvention as will be described below, the cured product will exhibitexcellent toughness due to the rubber backbone, and the molecular weightwill be higher than the rubber having the amino group and/or imino groupthat had been used for the starting material. The product will alsoexhibit long term stability since the highly reactive amino group and/orimino group is blocked, and the maleimide structure has relatively lowreactivity at ambient temperature.

Next, the curable composition of the present invention is described.

The curable composition of the present invention comprises the maleimideterminated rubber of the present invention and a resin and/or a rubberother than the maleimide terminated rubber of the present invention(hereinafter referred to as “the rubber other than the rubber of theinvention”).

The resin used in the curable composition of the present invention isnot particularly limited, and any conventional known resin may be usedas long as it has curability after incorporating in the composition.Exemplary resins include a resin which is curable by itself, a resinwhich is curable in the presence of a curing agent, a resin which iscurable by reacting with the maleimide terminated rubber of the presentinvention, a resin which is curable by reacting with other resin and/orother rubber in the curable composition of the present invention.

Exemplary such resins include epoxy resin (for example, liquid epoxyresin and solid epoxy resin), oxetane resin, and maleimide resin. Amongthese, epoxy resin has the merits of excellent dynamic properties andheat resistance while it also suffers from insufficient toughness.However, when epoxy resin is incorporated in the curable composition ofthe present invention, the toughness will be improved, and the strengthand other dynamic properties as well as the heat resistance will befurther improved. Therefore, the curable composition of the presentinvention containing the epoxy resin constitutes a preferred embodimentof the present invention.

The rubber other than the rubber of the invention used in the curablecomposition of the present invention is not particularly limited, andany conventional known rubber may be used as long as it has curabilitywhen incorporated in the composition. Exemplary such rubbers include arubber which is curable by itself, a rubber which is curable in thepresence of a curing agent (vulcanizing agent), a rubber which iscurable by reacting with the maleimide terminated rubber of the presentinvention, a rubber which is curable by reacting with other resin and/orother rubber in the curable composition of the present invention.

Exemplary such rubbers include a rubber having amino group and/or iminogroup on both ends of the molecule, diene rubber, and halogenatedrubber.

The curable composition of the present invention may contain two or moretypes of the above-described resins and/or rubbers other than the rubberof the invention.

The curable composition of the present invention may further contain oneor more resin and/or rubber other than the above-described resin and/orthe rubber other than the rubber of the invention in an amount that doesnot adversely affects the merit of the present invention.

The curable composition of the present invention may contain a curingagent.

The curing agent used may be at least one conventional known agentadequately selected depending on the above-described resin and/or therubber other than the rubber of the invention.

Among such curing agents, the preferred are those having a functionalgroup capable of reacting with the maleimide structure. When the curablecomposition of the present invention contains a curing agent having afunctional group capable of reacting with the maleimide structure, thecured product of the curable composition of the present invention willenjoy excellent properties including the strength owing to the reactionbetween the curing agent and the maleimide terminated rubber of thepresent invention that takes place in addition to the reaction betweenthe curing agent and the resin and/or the rubber other than the rubberof the invention.

The functional group capable of reacting with the maleimide structure isnot particularly limited. The functional group, however, is preferablyat least one member selected from a group consisting of amino group,imino group, thiol group, and diene structure in view of the reactivity.

Among these, the functional group capable of reacting with the maleimidestructure is preferably at least one member selected from a groupconsisting of amino group, imino group, and thiol group when epoxy resinis included in the curable composition of the present invention. Thesefunctional groups will react with the epoxy group in the epoxy resin,and also with the maleimide structure in the maleimide terminated rubberof the present invention, and therefore, the cured product of thecurable composition of the present invention will enjoy excellentproperties including the strength.

Exemplary curing agents having such functional group include1,3-benzenethiol, 4,4′-diaminodiphenylsulfone (DDS), m-phenylenediamine,diaminodiphenylmethane, and polysulfide resin having thiol group on itsend (for example, LP-3 manufactured by Thiokol Chem. Corp.).

Amount of the curing agent incorporated in the curable composition ofthe present invention is not particularly limited. Typically, the amountis preferably 10 to 100 parts by weight, and more preferably 10 to 50parts by weight per 100 parts by weight of the resin and/or the rubberother than the rubber of the invention.

The curable composition of the present invention may further contain anadditive such as plasticizer, filler, catalyst, solvent, UV absorbent,dye, pigment, flame retardant, reinforcing agent, antiaging agent,antioxidant, thixotropic agent, surfactant (including leveling agent),dispersant, dehydrator, anticorrosive, tackifier, and antistatic agentto the extent that the merits of the present invention are not adverselyaffected. These additives may be the one generally used in a rubbercomposition or a resin composition, and the additives may be used eitheralone or in combination of two or more.

The method for producing the curable composition of the presentinvention is not particularly limited, and the composition may beobtained, for example, by placing the essential and optional componentsas described above in the reaction vessel and fully kneading the mixtureunder reduced pressure by using an agitator such as blender.

The curable composition of the present invention has excellent toughnessafter curing as well as long term stability, and therefore, it is usedin various applications depending on the resin and/or the rubber otherthan the rubber of the invention. Typical preferable application iscomposite molded article comprising the curable composition of thepresent invention and other materials.

The material, the shape and the like of other materials used in thecomposite molded article of the present invention are not particularlylimited, and exemplary such materials include metal, resin moldedarticle, reinforcement fiber, and fiber reinforced plastic (FRP).

Among various applications, a preferred embodiment is use of thecomposite molded article for a prepreg. A prepreg is an intermediatematerial used in the molding obtainable by impregnating a reinforcementfiber such as carbon fiber in the matrix resin, and the prepreg is usedas a structural material in aircraft, automobile, and the like. Epoxyresin has been a typical material used for the matrix resin of theprepreg in view of the excellent dynamic properties, the heat resistanceand the like.

When the composite molded material of the present invention is formedinto a prepreg by using the curable composition of the present inventioncontaining the epoxy resin, an extremely favorable product will beproduced which enjoy excellent toughness in addition to the excellentdynamic properties (such as tensile strength and interlaminar shearstrength) and heat resistance.

The fiber used as the reinforcement in the prepreg of the presentinvention is not particularly limited, and exemplary fibers includecarbon fiber, glass fiber, and alamid fiber.

EXAMPLES

Next, the present invention is described in further detail by referringto the Examples, which by no means limit the scope of the presentinvention.

1. Preparation of the Maleimide Terminated Rubber of the PresentInvention

The maleimide terminated rubber of the present invention was produced bymixing 100 g of the amine terminated rubber represented by the aboveformula (1) (amine terminated liquid NBR, HYCAR AT polymer ATBN1300×45manufactured by Ube Industries, Ltd. having a weight average molecularweight of 3500 and an amine equivalent of 1900) and 10.8 g of4,4′-diphenylmethane bismaleimide (the amounts were such that maleimidestructure/imino group was 1.15) in 300 g of MEK and stirring the mixtureat room temperature for 5 hours, and then, at 70° C. for 2 hours to bereacted (see the reaction scheme, below).

The maleimide terminated rubber of the present invention represented bythe above formula (4) (wherein n represents the number of the recurringunits) had a weight average molecular weight of 15,500.

2. Preparation of the Curable Composition (1)

Example 1 and Comparative Example 1

The starting materials as described below were mixed at the weight ratioas shown in Table 1 to produce various curable compositions.

-   -   Liquid epoxy resin 1: ELM434 manufactured by Sumitomo Chemical        Co., Ltd.    -   Solid epoxy resin: Dicyclopentadiene epoxy resin, EPICLON        HP-7200 manufactured by Dainippon Ink and Chemicals,        Incorporated.    -   Liquid epoxy resin 2: Bisphenol A epoxy resin, YD-128        manufactured by Touto Kasei Corporation.    -   4,4′-diaminodiphenylsulfone (DDS): Seikacure-S manufactured by        Wakayama Seika Kogyo Co., Ltd.    -   MEK    -   Amine terminated rubber: The amine terminated rubber (amine        terminated liquid NBR, HYCAR AT polymer ATBN 1300×45,        manufactured by Ube Industries, Ltd.)

Maleimide terminated rubber: The maleimide terminated rubber representedby the formula (4) produced by the procedure as described above. TABLE 1Comparative Example 1 Example 1 Liquid epoxy resin 1 70 70 Solid epoxyresin 25 25 Liquid epoxy resin 2 5 5 DDS 31 31 MEK 100 100 Amineterminated rubber 10 Maleimide terminated rubber 103. Evaluation of Physical Properties (1)

The curable compositions produced in Example 1 and Comparative Example 1were evaluated for their physical properties as described below.

(1) Long Term Stability

The curable compositions produced in Example 1 and Comparative Example 1were stirred in MEK solvent at room temperature for 6 hours, and thestirring was continued for another 5 hours at reduced pressure and at60° C. to remove the solvent. The condition of the curable compositionswas visually observed, and the long term stability was evaluated byobserving whether curing of the curable compositions had occurred ornot.

(2) Appearance of the Cured Product

The curable compositions produced in Example 1 and Comparative Example 1were stirred in MEK solvent at room temperature for 6 hours, and thestirring was continued for another 5 hours at reduced pressure and at60° C. to remove the solvent. The curable compositions were then curedby allowing the compound to stand at 180° C. for 2 hours to therebyproduce the cured products. The cured products were visually observedfor their appearance.

(3) Glass Transition Temperature (T_(g))

The cured product was obtained by the procedure similar to theevaluation of the appearance of the cured product. The resulting curedproduct was evaluated for the glass transition temperature (T_(g)) usinga differential scanning calorimeter (DSC, model DSC2920, manufactured byTA Instrument Corporation).

The results of the evaluation of the physical properties are shown inTable 2. As demonstrated in Table 2, the curable composition of thepresent invention produced by using the maleimide terminated rubber ofthe present invention (Example 1) is superior in the long term stabilityand T_(g) compared to the case using the amine terminated rubber(Comparative Example 1). In the observation of the cured products, thecurable composition of the present invention (Example 1) was translucentindicating the absence of the phase separation between the rubber andthe resin. On the other hand, in the case using the amine terminatedrubber (Comparative Example 1), the composition was opaque indicatingthe occurrence of the phase separation. TABLE 2 Comparative Example 1Example 1 Long term stability Partial Stable curing Appearance of theOpaque Translucent cured product T_(g) (° C.) 187 1964. Preparation of the Curable Composition (2)

Examples 2 and 3 and Comparative Examples 2 and 3

The starting materials as described below were mixed at the weight ratioas shown in Table 3 to produce various curable compositions.

-   -   Polyfunctional epoxy resin: Tetrakis(glycidyloxy-phenyl)ethane,        Epikote 1031S, manufactured by Japan Epoxy Resins Co., Ltd.    -   Solid epoxy resin: Dicyclopentadiene epoxy resin, EPICLON        HP-7200 manufactured by Dainippon Ink and Chemicals,        Incorporated.    -   Liquid epoxy resin 2: Bisphenol A epoxy resin, YD-128        manufactured by Touto Kasei Corporation.    -   Liquid epoxy resin 3: Naphthalene epoxy resin, EPICLON HP-4032        manufactured by Dainippon Ink and Chemicals, Incorporated.    -   4,4′-diaminodiphenylsulfone (DDS): Seikacure S manufactured by        Wakayama Seika Kogyo Co., Ltd.    -   BF₃MEA: Trifluoromonoethylamine complex manufactured by Stella        Chemifa Corporation.    -   Maleimide terminated rubber: The maleimide terminated rubber        represented by the formula (4) produced by the procedure as        described above.

Carboxylated NBR: Nipol-1072 manufactured by ZEON Corporation, degree ofcarboxylation 5%. TABLE 3 Compara- Compara- tive tive Example ExampleExample Example 2 2 3 3 Polyfunctional epoxy 21 21 29 29 resin Solidepoxy resin 15 15 Liquid epoxy resin 2 32 32 28 28 Liquid epoxy resin 332 32 43 43 DDS 28 28 30 30 BF₃MEA 0.5 0.5 0.5 0.5 Maleimide terminated2 2 rubber Carboxylated NBR 2 25. Production of Fiber Reinforced Composite Material

The curable compositions produced in Examples 2 and 3 and ComparativeExamples 2 and 3 were used to produce the fiber reinforced compositematerials as described below.

The curable composition was coated on a release paper using a reverseroll coater to produce a resin film. A sheet of carbon fibers aligned inone direction (Torayca T800HB, manufactured by Toray Industries, Inc.,tensile modulus 294 GPa) was then sandwiched between two of the thusproduced resin films, and the fibers were impregnated with the resin byapplying pressure and heat to produce a unidirectional prepreg. Theresulting unidirectional prepreg had a carbon fiber weight of 196±5g/cm² and matrix resin ratio of 34% by weight.

10 sheets of the thus obtained unidirectional prepregs were stack one onanother with the carbon fibers aligned in the same direction, and thelaminate was heated by using an autoclave to 180° C. at a temperatureelevation rate of 2° C./minute, and the conditions of 180° C. and 0.59MPa were retained for 2 hours to thereby form fiber reinforced compositematerial sheet.

6. Evaluation of Physical Properties (2)

The thus produced unidirectional prepreg and the fiber reinforcedcomposite material were evaluated for physical properties by theprocedure as described below.

(1) Tack and Drape of the Unidirectional Prepreg

The as-produced two sheets of unidirectional prepregs were stack one onanother, and the prepregs were then manually separated to sensuallyevaluate the degree of tackiness (tack). The as-produced unidirectionalprepreg was also manually bent to sensually evaluate the presence ofdrape. The evaluations were carried out at 25° C.

The unidirectional prepregs which had been left in the room at 25° C.for 10 days after its production were also evaluated by the sameprocedure.

(2) Interlaminar Shear Strength of the Fiber Reinforced CompositeMaterial

Test pieces each having a length in the fiber direction of 20 mm, awidth in the direction vertical to the fiber direction of 10 mm, and athickness of 1.87 mm were cut out of the fiber reinforced compositematerial sheet, and bending test was carried out for these test piecesby using an autograph in accordance with the method of JIS K7203-1995.The bending test was carried out by three-point bending using a spanlength of 10 mm and a test speed of 1 mm/minute, and the interlaminarshear strength was calculated by the following formula. The bending testwas conducted at 25° C., 90° C., and 120° C., respectively.Interlaminar shear strength=load at break/(test piece width×test piecethickness)×¾(3) 90 Degree Tensile Strength of the Fiber Reinforced CompositeMaterial

Test pieces each having a length in the fiber direction of 25 mm, awidth in the direction vertical to the fiber direction of 250 mm, and athickness of 1.87 mm were cut out of the fiber reinforced compositematerial sheet, and tensile test at 90 degrees in relation to the fiberdirection was carried out for these test pieces by using an autograph inaccordance with the method of ASTM D3039. The tensile test was carriedout at grip length of 50 mm and test speed of 0.5 mm/minute, and the 90degree tensile strength was calculated by the following formula. Thebending test was conducted at 25° C.90 degree tensile strength=load at break/(test piece width×test piecethickness)

The results of the evaluation of the physical properties for theunidirectional prepreg are shown in Table 4. As demonstrated in Table 4,the curable composition of the present invention produced by using themaleimide terminated rubber of the present invention (Examples 2 and 3)is superior in the tackiness and drape and their long term stabilitywhen prepared into a prepreg compared to the case using carboxylated NBR(Comparative Examples 2 and 3) exhibiting inferior long term stability.

The results of the evaluation of the physical properties for fiberreinforced composite material are shown in Table 5. As demonstrated inTable 5, the curable composition of the present invention produced byusing the maleimide terminated rubber of the present invention (Examples2 and 3) is superior in the strength, and in particular, in the strengthat high temperature when prepared into the fiber reinforced compositematerial compared to the case using carboxylated NBR (ComparativeExamples 2 and 3). TABLE 4 Compara- Compara- tive tive Example ExampleExample Example 2 2 3 3 Tack As produced Detected Detected DetectedDetected 10 days after Detected Not Detected Not the production DetectedDetected Drape As produced Present Present Present Present 10 days afterPresent Not Present Not the production Present Present

TABLE 5 Compara- Compara- tive tive Example Example Example Example 2 23 3 Interlaminar shear strength (MPa)  25° C. 120.7 112.1 121.8 113.7 90° C. 89.5 86.3 94.5 88.0 120° C. 80.7 73.2 85.1 77.4 90 degree 85.276.3 85.5 69.4 tensile strength (MPa)

1. A maleimide terminated rubber having maleimide structure on both endsof the molecule obtainable by reacting a rubber having amino groupand/or imino group on both ends of the molecule and a bismaleimidecompound.
 2. A curable composition comprising the maleimide terminatedrubber of claim 1 and a resin and/or a rubber other than the maleimideterminated rubber.
 3. The curable composition according to claim 2further comprising a curing agent having a functional group capable ofreacting with the maleimide structure.
 4. The curable compositionaccording to claim 3 wherein the functional group is at least one memberselected from a group consisting of amino group, imino group, thiolgroup, and diene structure.
 5. The curable composition according toclaim 3 wherein the resin is epoxy resin, and the functional group is atleast one member selected from a group consisting of amino group, iminogroup, and thiol group.
 6. A composite molded article wherein thecurable composition of claim 1 is used.
 7. A composite molded articlewherein the curable composition of claim 2 is used.
 8. A compositemolded article wherein the curable composition of claim 3 is used.
 9. Acomposite molded article wherein the curable composition of claim 4 isused.
 10. A composite molded article wherein the curable composition ofclaim 5 is used.
 11. A prepreg wherein the curable composition of claim5 is used.